Process for producing electrode for electroluminescence

ABSTRACT

The present invention provides an electrode for electroluminescence for use in electronic devices including organic electroluminescent devices and a process for producing the same, in which interfacial electric characteristics, such as work functions, can be easily controlled. The process for producing an electrode for electroluminescence comprises the step of diffusing an additive element for an electrode into the electrode and/or the step of developing surfactant properties of the additive element for an electrode.

TECHNICAL FIELD

[0001] The present invention relates to an electrode, forelectroluminescence, having controlled interfacial electriccharacteristics and a process for producing the same. More particularly,the present invention relates to an electrode for electroluminescencefor use in various electronic devices including organicelectroluminescent devices and a process for producing the same.

BACKGROUND ART

[0002] In the electroluminescence industry, what has hitherto beendemanded is the control of interfacial electric characteristics ofelectrodes for electroluminescence. Interfacial electric characteristicsof electrodes, such as work function, affect the efficiency of chargeinjection into a luminescent layer and greatly influence luminescenceefficiency. For this reason, an attempt has hitherto been made tocontrol the relationship between work functions of electrode materials.In this case, a complicated process, for example, involving intentionalprovision of interlayer materials has been adopted rather than theadoption of control utilizing diffusion and surfactant properties. Theprovision of very thinly controlled layers of these materials, however,disadvantageously complicates the production process and increases theproduction cost.

DISCLOSURE OF THE INVENTION

[0003] An object of the present invention is to provide an electrode forelectroluminescence which can easily control interfacial electriccharacteristics such as work functions and a process for producing thesame and particularly to provide an electrode for electroluminescencefor use in electronic devices including organic electroluminescentdevices and a process for producing the same.

[0004] With a view to attaining the above object, the present inventorhas made intensive and extensive studies and, as a result, has foundthat the interfacial electric characteristics, such as work functions,of an electrode for electroluminescence can be easily controlled bymigration-depositing, in an island form or in a layer form, an additiveelement for an electrode, for example, tin (Sn), antimony (Sb), lithium(Li), or cesium (Cs), on the surface of the electrode forelectroluminescence through the utilization of diffusion and/orsurfactant effect of the additive element through grain boundaries ofthe electrode for electroluminescence and, further, optionally surfacemodifying the electrode. This has led to the completion of the presentinvention.

[0005] Thus, according to one aspect of the present invention, there isprovided a process for producing an electrode for electroluminescence,comprising the step of diffusing an additive element for an electrodeinto the electrode and/or developing surfactant properties of theadditive element.

BEST MODE FOR CARRYING OUT THE INVENTION Step of Diffusion and Step ofDeveloping Surfactant Properties

[0006] An electrode, particularly an electrode formed by a vacuum thinfilm formation method has the following features. Specifically, in thiselectrode, grain boundaries are likely to be formed within theelectrode. When a metal is present within the electrode or at theinterface of the substrate and the electrode, the metal migrates throughthe grain boundaries to the surface of the electrode and is deposited onthe outermost surface of the electrode. Examples of such phenomenainclude diffusion, that is, a phenomenon wherein the material migratesfrom a higher material concentration site to a lower materialconcentration site to homogenize the material concentration, andsurfactant effect, that is, a phenomenon wherein, when a material B isdeposited on a surface formed of a material A, which is different fromthe material B, the material A migrates through the material B,deposited on the material A, to the surface of the material B to form asurface formed of the material A. The migration-deposited metal grainsof the same type are likely to aggregate together.

[0007] According to the present invention, an electrode forelectroluminescence having controlled interfacial electriccharacteristics, such as a controlled work function of the surface, isprovided by controlling the migration-depositing of an element (anadditive element for an electrode) onto the surface of the electrodethrough the utilization of these properties and optionally conductingpost-treatment. The control of the work function of the surface of theelectrode for electroluminescence by this method can advantageouslysimplify the production process and can reduce the cost.

[0008] The development of the above diffusion effect and surfactanteffect can be accelerated by the application of various types of energy.Examples of energy usable herein include those obtained by heat(preferably at 200° C. or above) irradiation, ultrasonic waveirradiation, electromagnetic wave irradiation, plasma irradiation, andion irradiation.

Additive Element for Electrode

[0009] The additive element for an electrode used in the presentinvention is not limited so far as the additive element can develop thediffusion effect and/or surfactant effect in the electrode forelectroluminescence and the additive element or a compound of theadditive element, such as an oxide of the additive element, can changethe interfacial electric characteristics of the surface of the electrodefor electroluminescence.

[0010] Regarding such additive elements for an electrode, examples ofadditive elements usable for an anode include elements having a largerwork function than the anode or hole-rich oxides. Examples of additiveelements usable for a cathode include elements having a smaller workfunction than the cathode or electron-rich oxides, and elements whichcan be converted to nitrides.

[0011] Specific examples of additive elements usable for the formationof the anode include tin (Sn), antimony (Sb), gold (Au), cobalt (Co),iridium (Ir), osmium (Os), palladium (Pd), platinum (Pt), tungsten (W),arsenic (As), nickel (Ni), copper (Cu), iron (Fe), bismuth (Bi),praseodymium (Pr), and thallium (Tl). Specific examples of additiveelements usable for the formation of the cathode include cerium (Ce),rubidium (Rb), cesium (Cs), lithium (Li), sodium (Na), calcium (Ca),magnesium (Mg), europium (Eu), erbium (Er), ytterbium (Yb), yttrium (Y),barium (Ba), strontium (Sr), zirconium (Zr), and titanium (Ti).

[0012] The above elements may be used for the formation of the electrodeby various methods. Examples of methods usable herein include: theincorporation of the additive element as a component of the material forthe electrode for electroluminescence; the formation of the electrodefor electroluminescence on a layer of the additive element; and theformation of a layer of the additive element on the electrode forelectroluminescence. Methods usable for the formation of the layer ofthe additive element for the electrode include sputtering, ion plating,vapor deposition, CVD, and MBE.

[0013] When the electrode for electroluminescence is a transparentelectrode and, at the same time, when a layer of the additive elementfor the electrode is provided, what is required of the layer of theadditive element is not to sacrifice the transparency of the electrodefor electroluminescence. Specifically, for example, the layer of theadditive element preferably has a light (for example, 550 nm light)transmittance of not less than 80%.

Electrode for Electroluminescence

[0014] The electrode for electroluminescence according to the presentinvention comprises an electrode for electroluminescence and, providedon the electrode, a layer containing an additive element for theelectrode. The layer containing an additive element for the electrode isnot limited so far as the layer has been formed by the step of diffusionand/or the step of developing surfactant properties.

[0015] Materials of electrodes for electroluminescence include, forexample, ITO and platinum (Pt).

[0016] In an embodiment of the electrode for electroluminescenceaccording to the present invention, the additive element for anelectrode is present within the electrode and on one side (lower layer)of the electrode, and the concentration of the additive element in thesurface of the electrode is higher than that of the additive elementwithin the electrode.

[0017] When the electrode for electroluminescence according to thepresent invention is used as an anode, preferably, a layer containing anadditive element for an electrode having a larger work function than theanode is provided on the anode. On the other hand, when the electrodefor electroluminescence according to the present invention is used as acathode, preferably, a layer containing an additive element for anelectrode having a smaller work function than the cathode is provided onthe cathode.

[0018] The cathode electrode is transparent as viewed from the cathodeside and has rectification capability.

[0019] In the formation of the electrode for electroluminescenceaccording to the present invention, conventional methods may be used,and examples thereof include sputtering, ion plating, and vapordeposition.

Post-Treatment of Electrode for Electroluminescence

[0020] In the electrode for electroluminescence according to the presentinvention, if necessary, after the migration-depositing of the additiveelement for an electrode, post-treatment, such as the formation of acompound of the additive element or surface conditioning, may be carriedout.

[0021] Examples of preferred post-treatment usable herein include oxygenplasma treatment, chlorine plasma treatment, nitrogen plasma treatment,ammonia plasma treatment, fluorine plasma treatment, UV treatment, ozonetreatment, and heat annealing treatment.

[0022] More specifically, in the anode, when platinum, antimony, gold,tin, nickel, copper, iron, bismuth, praseodynium, thallium, etc. areused as the additive element for the electrode, oxygen plasma treatmentmay be mentioned as preferred post-treatment; and when antimony and ironare used as the additive element for the electrode, chlorine plasmatreatment is preferred as the post-treatment. On the other hand, in thecathode, when barium, calcium, strontium, yttrium, etc. are used as theadditive element for the electrode, oxygen plasma treatment is preferredas the post-treatment; when zirconium, titanium, etc. are used as theadditive element for the electrode, nitrogen plasma treatment or ammoniaplasma treatment is preferred as the posttreatment; and when lithiumetc. is used as the additive element for the electrode, fluorine plasmatreatment is preferred as the post-treatment.

Where Electrode for Electroluminescence is ITO

[0023] In a preferred embodiment of the present invention, the electrodefor electroluminescence is an ITO electrode.

[0024] In this case, specifically, for example, when the content of tinin an ITO target is high, that is, when the content of SnO₂ in ITO ishigh (for example, the content of SnO₂ in ITO is preferably not lessthan 4% by weight and not more than 20% by weight and, in the case ofsputtering, more preferably 13% by weight), the migration-depositing oftin on the surface of the electrode is accelerated. Upon subsequentoxidation, for example, by oxygen plasma treatment, UV treatment, orozone treatment, an SnO₂ layer can be formed on the outermost surface ofthe electrode to increase work function, and good interfacial electricconnection between ITO and a hole transport layer can be realized.

[0025] Further, the work function can be increased by a method wherein,after the deposition of tin or antimony on a substrate, a thin-film ITOelectrode is formed to migration-deposit tin or antimony on the surfaceof the thin-film electrode, and the above post-treatment is then carriedout. The application of proper energy, such as heat, at the time of ITOthin film formation accelerates grain boundary diffusion and developmentof surfactant effect of tin or other metal having surfactant effectwhich permits tin to be migration-deposited on the outermost surface ofthe electrode. Subsequent proper post-treatment in the same manner asdescribed above can increase the work function.

[0026] The content of SnO₂ in the ITO electrode according to the presentinvention is preferably 2 to 20% by weight. More preferably, when ITO isformed by sputtering, the content of SnO₂ in ITO is about 13% by weight;when ITO is formed by ion plating, the content of SnO₂ in ITO is about8% by weight; and when ITO is formed by vapor deposition, the content ofSnO₂ in ITO is about 15% by weight.

[0027] In the ITO electrode according to the present invention,preferably, the concentration of SnO₂ in the surface of the ITOelectrode is higher than the concentration of SnO₂ within the ITOelectrode.

Electroluminescent Device

[0028] The electrode for electroluminescence according to the presentinvention is preferably used as an anode and/or a cathode in anelectroluminescent device, particularly an organic electroluminescentdevice. Preferably, the electroluminescent device comprises at least ananode, an electroluminescent layer provided on the anode, and a cathodeprovided on the electroluminescent layer. More preferably, theelectroluminescent device comprises at least an anode, a hole transportlayer provided on the anode, an electroluminescent layer provided on thehole transport layer, and a cathode provided on the electroluminescentlayer.

[0029] For example, when the electrode for electroluminescence accordingto the present invention is used as an anode in an organicelectroluminescent device, the work function of the surface of the anodecan be enhanced at the interface between the anode and the holetransport layer to reduce the gap in work function between the surfaceof the anode and the hole transport layer. As a result, the efficiencyof hole injection can be improved to improve luminescencecharacteristics of the organic electroluminescence. The luminescencecharacteristics of the organic electroluminescence can also be improvedby forming a hole-rich material on the surface of the anode. On theother hand, in the case of use of the electrode for electroluminescenceaccording to the present invention as the cathode in the organicelectroluminescent device, the work function of the surface of thecathode is reduced, the electric characteristics of the connectioninterface are improved, and the efficiency of electron injection can berealized to improve luminescence characteristics of the organicelectroluminescence.

[0030] Embodiments of electroluminescent devices using the electrode forelectroluminescence according to the present invention will bedescribed.

[0031] In an embodiment, an organic electroluminescent device has adevice structure of substrate/X/ITO/hole transport layer/luminescentlayer/cathode wherein X represents an additive element for an electrode,such as tin or antimony. In this electroluminescent device, after theformation of the X layer, the ITO layer is formed. In the formation ofthe ITO layer, proper energy is applied to accelerate the diffusion of Xinto the surface of ITO. Thereafter, proper post-treatment is carriedout to control the work function, whereby good electrically connectedinterface between ITO (anode) and the hole transport layer can berealized.

[0032] In another embodiment, an organic electroluminescent device has adevice structure of substrate/ITO/hole transport layer/luminescentlayer/cathode. In this electroluminescent device, the optimization ofthe content of SnO₂ in an ITO target can control the content of tin inthe surface of ITO, and subsequent proper post-treatment can controlinterfacial electric characteristics, such as work function, wherebygood electrically connected interface between ITO (anode) and the holetransport layer can be realized.

[0033] In a further embodiment, an organic electroluminescent device hasa device structure of substrate/X/cathode/luminescent layer/holetransport layer/ITO wherein X represents an additive element for anelectrode, such as cerium (Ce) or rubidium (Rb). In thiselectroluminescent device, after the formation of the X layer, thecathode layer is formed. In the formation of the cathode, proper energyis applied to accelerate the diffusion of X into the surface of thecathode. Thereafter, proper post-treatment is carried out to controlinterfacial electric characteristics, such as work function, wherebygood electrically connected interface between the cathode and theluminescent layer can be realized.

Interfacial Electric Characteristics

[0034] In the electrode for electroluminescence according to the presentinvention, interfacial electric characteristics, such as work function,can be controlled to a suitable value. The work function can bemeasured, for example, an UV photoelectron spectroscopic device (forexample, AC-1, manufactured by RIKEN KEIKI CO., LTD) and affectsphotoelectric effect and the like.

EXAMPLES Example 1

[0035] An about 10 nm-thick platinum (Pt) layer was formed by sputteringon a glass substrate. A 1,500 angstrom-thick ITO layer was then formedthereon by sputtering. At the time of the formation of the ITO layer, anattempt to diffuse platinum into the surface of the ITO layer was madeby heating the substrate at about 250° C. As a result,migration-depositing of platinum on the surface of ITO was observed, andthe work function value of the surface of the electrode was increased.Thereafter, a hole transport layer and a luminescent layer were formedby spin coating, and a 20 nm-thick calcium (Ca) electrode and a 2,000angstrom-thick silver (Ag) electrode were formed by vapor depositionusing a mask to prepare an organic electroluminescent device. Thisorganic electroluminescent device was evaluated for luminescencecharacteristics and, as a result, was found to have improved brightness.

Example 2

[0036] An about 10 nm-thick antimony (Sb) layer was formed by sputteringon a glass substrate. A 1,500 angstrom-thick ITO layer was then formedthereon by sputtering. At the time of the formation of the ITO layer, anattempt to diffuse antimony into the surface of the ITO layer was madeby heating the substrate at about 250° C. As a result,migration-depositing of antimony on the surface of ITO was observed. Theassembly was then subjected to chlorine plasma treatment. Upon thistreatment, it was confirmed that SbCl₂ was present on the surface of theassembly. Further, a hole transport layer and a luminescent layer wereformed by spin coating, and a 20 nm-thick calcium electrode and a 2,000angstrom-thick silver electrode were formed by vapor deposition using amask to prepare an organic electroluminescent device. This organicelectroluminescent device was evaluated for luminescence characteristicsand, as a result, was found to have improved brightness.

Example 3

[0037] Tin (Sn) pieces were put on an In₂O₃ target, and sputtering wascarried out on a glass substrate. In this case, the area of the tinpieces and the number of the tin pieces were controlled. Thus, a 1,500angstrom-thick In₂O₃ layer having an SnO₂ content of 0% and 1,500angstrom-thick ITO layers respectively having SnO₂ contents of 5%, 10%,13%, and 20% were formed on the substrate. At the time of the formationof the ITO layers, the substrate was heated at about 250° C. toaccelerate diffusion of tin into and segregation of tin on the surfaceof ITO. As a result, it was found that the level of segregation of tinon the surface of ITO was increased in increasing order of SnO₂ content,that is, in the following order: 5%, 10%, 13%, and 20%. Thereafter,oxygen plasma treatment or UV irradiation treatment (in air) was carriedout. For both the treatments, the work function of the surface of theITO was increased in increasing order of SnO₂ content, that is, in thefollowing order: 5%, 10%, 13%, and 20%. Thereafter, a hole transportlayer and a luminescent layer were formed by spin coating, and a 20nm-thick calcium electrode and a 2,000 angstrom-thick silver electrodewere formed by vapor deposition using a mask. The luminescencecharacteristics of the organic electroluminescent devices wereevaluated. As a result, the organic electroluminescent device using theITO layer having an SnO₂ content of 13% had the highest brightness.Accordingly, a sputtering target having such a composition that thevalue of SnO₂/(In₂O₃+SnO₂) was 13% was prepared. A 1,500 angstrom-thickITO layer was formed by sputtering at 250° C. using this target. Thecontent of SnO₂ in the ITO layer was measured and found to be about 13%.In the same manner as described above, oxygen plasma treatment wascarried out, and an organic electroluminescent device was then preparedusing the electrode thus obtained. The organic electroluminescent devicewas found to have improved luminescence brightness over a conventionalorganic electroluminescent device having such a sputtering targetcomposition that the value of SnO₂/(In₂O₃+SnO₂) was 10%. The sameexperiment as described above was carried out except that the ITO layerwas formed by ion plating or vapor deposition instead of sputtering. Asa result, for the organic electroluminescent devices thus obtained, themaximum luminescence brightness was obtained in such a materialcomposition that the value of SnO₂/(In₂O₃+SnO₂) was 8% for the ionplating and in such a material composition that the value ofSnO₂/(In₂O₃+SnO₂) was 15% for the vapor deposition.

[0038] As is apparent from the foregoing description, according to thepresent invention, the surface electric characteristics of the electrodefor electroluminescence can be improved. When the electrode forelectroluminescence according to the present invention is used in anorganic electroluminescent device, increased luminescence efficiency ofthe organic electroluminescent device can be realized. Further, when ITOis used in an electrode for electroluminescence, increasing the contentof SnO₂ in the surface of the electrode can be also expected to attainsuch an effect that the acid resistance of the electrode can beimproved, the diffusion of impurity ions contained in an ITO electrodeor a glass substrate can be suppressed, and, thus, the characteristicsof the electrode can be improved.

What is claimed is:
 1. A process for producing an electrode forelectroluminescence, comprising the step of diffusing an additiveelement for an electrode into said electrode and/or the step ofdeveloping surfactant properties of the additive element for anelectrode.
 2. The process for producing an electrode forelectroluminescence according to claim 1, wherein the step of diffusionand/or the step of development of surfactant properties are the step ofapplying, to said electrode, energy obtained by heat irradiation,ultrasonic wave irradiation, electromagnetic wave irradiation, plasmairradiation, or ion irradiation method.
 3. The process for producing anelectrode for electroluminescence according to claim 1, which comprisesthe steps of: forming the electrode for electroluminescence on a layerof the additive element for an electrode; and diffusing the additiveelement for an electrode into said electrode and/or developingsurfactant properties of the additive element for an electrode.
 4. Theprocess for producing an electrode for electroluminescence according toclaim 1, which comprises the steps of: forming a layer of the additiveelement for an electrode on the electrode for electroluminescence; anddiffusing the additive element for an electrode into said electrodeand/or developing surfactant properties of the additive element for anelectrode.
 5. The process for producing an electrode forelectroluminescence according to claim 1, wherein the additive elementfor an electrode is a constituent of a material for the electrode forelectroluminescence.
 6. The process for producing an electrode forelectroluminescence according to claim 1, wherein the electrode forelectroluminescence is formed by a method selected from sputtering, ionplating, vapor deposition, CVD, and MBE.
 7. The process for producing anelectrode for electroluminescence according to claim 1, wherein theelectrode for electroluminescence is transparent and is an anode and theadditive element for an electrode used in the formation of the anode isselected from the group consisting of tin (Sn), antimony (Sb), gold(Au), cobalt (Co), iridium (Ir), osmium (Os), palladium (Pd), platinum(Pt), tungsten (W), arsenic (As), nickel (Ni), copper (Cu), iron (Fe),bismuth (Bi), praseodymium (Pr), and thallium (Tl).
 8. The process forproducing an electrode for electroluminescence according to claim 1,wherein the electrode for electroluminescence is transparent and is acathode and the additive element for an electrode used in the formationof the cathode is selected from the group consisting of cerium (Ce),rubidium (Rb), cesium (Cs), lithium (Li), sodium (Na), calcium (Ca),magnesium (Mg), europium (Eu), erbium (Er), ytterbium (Yb), yttrium (Y),barium (Ba), strontium (Sr), zirconium (Zr), and titanium (Ti).
 9. Theprocess for producing an electrode for electroluminescence according toclaim 1, wherein the electrode for electroluminescence is an ITOelectrode and the content of SnO₂ in the ITO electrode is 2 to 30% byweight.
 10. The process for producing an electrode forelectroluminescence according to claim 9, wherein the ITO electrode isformed by sputtering using a material having an SnO₂ content of 2 to 13%by weight.
 11. The process for producing an electrode forelectroluminescence according to claim 9, wherein the ITO electrode isformed by ion plating using a material having an SnO₂ content of 2 to 8%by weight.
 12. The process for producing an electrode forelectroluminescence according to claim 9, wherein the ITO electrode isformed by vapor deposition using a material having an SnO₂ content of 2to 15% by weight.
 13. The process for producing an electrode forelectroluminescence according to claim 1, which further comprises, afterthe step of diffusion and/or the step of development of surfactantproperties, the step of subjecting the electrode to oxygen plasmatreatment, chlorine plasma treatment, nitrogen plasma treatment, ammoniaplasma treatment, fluorine plasma treatment, UV treatment, ozonetreatment, or heat annealing treatment.
 14. A process for producing anelectroluminescent device comprising at least an anode, anelectroluminescent layer, and a cathode, said process comprising thesteps of: forming an anode; forming an electroluminescent layer on theanode; and forming a cathode on the electroluminescent layer, said anodeand/or said cathode being formed by the process according to claim 1.15. An electrode for electroluminescence, said electrode containing anadditive element for an electrode, wherein the additive element for anelectrode is present within and one side of the electrode forelectroluminescence, and the concentration of the additive element inthe surface of the electrode is higher than the concentration of theadditive element within the electrode.
 16. The electrode forelectroluminescence according to claim 15, wherein the electrode forelectroluminescence is an anode and a layer, containing an additiveelement for an electrode, having a larger work function than the anodeis provided on the anode.
 17. The electrode for electroluminescenceaccording to claim 16, wherein the electrode for electroluminescence isan ITO electrode and the concentration of SnO₂ in the surface of the ITOelectrode is higher than the concentration of SnO₂ within the ITOelectrode.
 18. The electrode for electroluminescence according to claim15, wherein the electrode for electroluminescence is a cathode and alayer, containing an additive element for an electrode, having a smallerwork function than the cathode is provided on the cathode.
 19. Anelectroluminescent device comprising the electrode forelectroluminescence according to claim 15.